A normal human heart pumping pattern is called a sinus rhythm, and the pattern is regulated by the body's biological pacemaker within the upper right chamber of the heart, which is commonly referred to as the right atrium. This natural pacemaker, which is generally referred to as the sinoatrial (“SA”) node, sends electrical signals to the right and left ventricular muscles in the lower chambers of the heart. The ventricular muscles then implement the pumping action under the control of the SA node. The right ventricular muscle pumps blood to the lungs for oxygenation, and the left ventricular muscle pumps the oxygenated blood to various parts of the body.
In certain circumstances, the normal or sinus heartbeat rhythm may be adversely affected as a result of some type of malfunction in the heart's electrical control system. When this type of malfunction occurs, an irregular heartbeat may result, causing the ventricular muscles to pump ineffectively, thus reducing the amount of blood pumped to the body. This irregular heartbeat is generally referred to as an arrhythmia.
A particularly serious arrhythmia is known as ventricular fibrillation (“VF”), which is a malfunction characterized by rapid, uncoordinated cardiac movements replacing the normal contractions of the ventricular muscles. In this event, the ventricular muscles are not able to pump blood out of the heart, and there is no initiation of a coordinated heartbeat, leading to death within minutes. VF rarely terminates spontaneously, and its unpredictability exacerbates the clinical problem of treating patients in sudden cardiac arrest. Physicians emphasize the critical importance of rapid therapeutic intervention to prevent the loss of life.
Defibrillators are devices for providing life-saving electrical therapy to persons experiencing an irregular heat beat, such as VF. A defibrillator provides an electrical stimulus to the heart in an attempt to convert the irregular heat beat to a normal sinus rhythm. One commonly used type of defibrillator is the external defibrillator, which sends electrical pulses to the patient's heart through external electrodes applied to the patient's chest. External defibrillators may be manually operated (as are typically used in hospitals by medical personnel), semi-automatic, semi-automated, fully automatic, or fully automated devices, allowing them to be used in any location where an unanticipated arrhythmic event may occur. In practice, defibrillation pulses are administered to the patient when necessary, either in the judgment of medical personnel or as recommended by automated interpretive algorithms (“Shock Advisory Systems”), and therapeutic protocols may further recommend that cardio-pulmonary resuscitation (“CPR”), assisted ventilation, and/or supplemental drug therapies, among others, be administered between consecutive defibrillation pulses. CPR includes the delivery of chest compressions to the patient (to stimulate blood flow). Ventilation includes the delivery of air to the lungs by mechanical or physiologic means (e.g., mouth to mouth ventilation). Drugs may be delivered to stimulate cardiac tissue or to inactivate inhibiting or damaging substances in the heart, enhancing the effectiveness of defibrillator pulses.
The heart responds to defibrillation pulses in different ways depending upon how long the heart has been without perfusion. The prior art includes techniques that determine whether the heart is in a state that requires defibrillation therapy, and techniques that analyze the VF waveform to predict the responsiveness of the heart to defibrillation. These latter techniques are only able to make predictions based on statistical correlations between characteristics of the electrocardiogram (“ECG”) signal and the probability of successful defibrillation in a population exhibiting similar measurements. As such, the predictions can be inaccurate and the results can be corrupted by artifact.
Accordingly, it is desirable to have a system for assessing the likelihood of defibrillation therapy success that addresses the shortcomings of prior art systems and methods. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.